A new method to quantitatively control oxygen fugacity in externally heated pressure vessel experiments

Oxygen fugacity (fO(2)) is a fundamental variable affecting phase equilibrium in magmas, and in externally heated pressure vessel experiments it is typically controlled by using redox buffer assemblages. However, these do not allow fine enough resolution; for example, most arc magmas fall between the fO(2 )imposed by the neighboring Ni-NiO and Re-ReO2 buffers and so does the transition of S2- to S6+ in magmas. Here we propose a new method to quantitatively impose f O-2 in hydrous high-P-T experiments in molybdenum hafnium carbide (MHC) pressure vessels by admixing small amounts of hydrogen into the Ar pressure medium. The thermodynamic calculation procedure used to determine the initial amount of hydrogen to be loaded to constrain desired fO(2) values was verified by CoPd alloy redox sensor experiments to be accurate within +/- 0.3 log units for the pressure (P) - temperature (T) range of 940-2060 bar and 800-1100 degrees C. As hydrogen can be slowly lost from the pressure medium due to diffusion through the vessel walls at high T, we also determined the hydrogen permeability of the MHC alloy as a function of T. The such-obtained hydrogen permeability equation for the MHC alloy can be used to determine the rate of fO(2) increase for any MHC pressure vessel configuration. As the rate of fO(2) increase is slow (e.g., 0.36 log units per day in our setup at T = 1000 degrees C), we propose that H2 addition to the Ar pressure medium is an effective way to accurately impose fO(2) in many types of experiments conducted in MHC vessels allowing experimentation up to T = 1200 degrees C and P = 300 MPa.